Exploring the Biological Contributions to Human Health: Does Sex Matter?

It's obvious why only men develop prostate cancer and why only women get ovarian cancer. But it is not obvious why women are more likely to recover language ability after a stroke than men or why women are more apt to develop autoimmune diseases such as lupus.

Sex differences in health throughout the lifespan have been documented. Exploring the Biological

Overview

It's obvious why only men develop prostate cancer and why only women get ovarian cancer. But it is not obvious why women are more likely to recover language ability after a stroke than men or why women are more apt to develop autoimmune diseases such as lupus.

Sex differences in health throughout the lifespan have been documented. Exploring the Biological Contributions to Human Health begins to snap the pieces of the puzzle into place so that this knowledge can be used to improve health for both sexes. From behavior and cognition to metabolism and response to chemicals and infectious organisms, this book explores the health impact of sex (being male or female, according to reproductive organs and chromosomes) and gender (one's sense of self as male or female in society).

Exploring the Biological Contributions to Human Health discusses basic biochemical differences in the cells of males and females and health variability between the sexes from conception throughout life. The book identifies key research needs and opportunities and addresses barriers to research.

Exploring the Biological Contributions to Human Health will be important to health policy makers, basic, applied, and clinical researchers, educators, providers, and journalists-while being very accessible to interested lay readers.

The explosion in the growth of new biological information over the past decade has made it increasingly apparent that many normal physiological functions-and, in many cases, pathological functions-are influenced either directly or indirectly by sex-based differences in biology (also referred to throughout this report as sex differences).

In recent years, considerable attention has been given to the differences and similarities between females and males (1) at the societal level by researchers evaluating how individual behaviors, lifestyles, and surroundings affect one's biological development and health and (2) at the level of the whole organism by clinicians and applied researchers investigating the component organs and systems of humans. However, scientists have paid much less attention to the direct and intentional study of these differences at the basic cellular and molecular levels. Where data are available, they have often been a by-product of other areas of research. Historically, the research community assumed that beyond the reproductive system such differences do not exist or are not relevant.

Scientific evidence of theimportance of sex differences throughout the life span abounds. Investigators are now positioned to take this work to the next level, at which the mechanisms and origins of such differences can be explored. This will allow scientists and clinicians to understand the implications of these differences for human health. The critical questions to be answered are

How can information on sex differences be translated into preventive, diagnostic, and therapeutic practice?

How can the new knowledge about and understanding of biological sex differences and similarities most effectively be used to positively affect patient outcomes and improve health and health care?

SCOPE OF THE REPORT

In November 1999, the Institute of Medicine (IOM) formed the Committee on Understanding the Biology of Sex and Gender Differences in response to combined requests from a consortium of public and private sponsors. The committee members brought expertise from a broad array of disciplines in basic and applied biomedical research. In general, the sponsors asked the committee to evaluate and consider the current understanding of sex differences and determinants at the biological level. Specifically, they asked that the following issues be addressed:

the knowledge base on and research priorities for animal and cellular models that could be used to determine when sex and gender differences exist and when they are relevant to biological functioning at the cellular, developmental, organ, organismal, and behavioral levels;

current and potential barriers to the conduct of valid and productive research on sex and gender differences and their determinants, including ethical, financial, sociological, and scientific factors; and

strategies that can be used to overcome such barriers and promote the acceptance of this research by the scientific community and the general public.

The committee was not charged with the task of preparing a definitive text on all known differences and similarities between the sexes but, rather, was charged with considering factors and traits that characterize and differentiate males and females across the life span and that underlie sex differences in health (including genetic, biochemical, physiological, physical, and behavioral elements). Thus, the focus of this report is on sex-based differences, versus similarities, as they are more likely to successfully demonstrate the need for further research and lead to greater understanding of the significance of sex in human biology and health. Moreover, despite the influence of pregnancy, parity, and parenthood on the manifestation of some diseases and health outcomes, this report does not directly address these issues, as they are deserving of separate and more in-depth attention.

On the basis of its review, the committee arrived at a series of findings and conclusions and developed recommendations that are designed to facilitate scientific endeavors in this area, take advantage of new opportunities in basic and applied research, and fill identified research gaps.

OVERARCHING CONCLUSIONS

Three common, recurring messages emerged as the committee addressed its primary task (reviewing and evaluating the current state of knowledge about sex differences in health and illness and scientific evidence related to sex differences in health and illness) and as it met with scientific experts across diverse disciplines.

Sex matters. Sex, that is, being male or female, is an important basic human variable that should be considered when designing and analyzing studies in all areas and at all levels of biomedical and health-related research. Differences in health and illness are influenced by individual genetic and physiological constitutions, as well as by an individual's interaction with environmental and experiential factors. The incidence and severity of diseases vary between the sexes and may be related to differences in exposures, the routes of entry and the processing of a foreign agent, and cellular responses. Although in many cases these sex differences can be traced to the direct or indirect effects of hormones associated with reproduction, differences cannot be solely attributed to hormones.

The study of sex differences is evolving into a mature science. There is now sufficient knowledge of the biological basis of sex differences to validate the scientific study of sex differences and to allow the generation of hypotheses. The next step is to move from the descriptive to the experimental and to establish the conditions that must be in place to facilitate and encourage the scientific study of the mechanisms and origins of sex differences. Naturally occurring variations in sexual differentiation and development can provide unique opportunities to obtain a better understanding of basic differences and similarities between and within the sexes.

Barriers to the advancement of knowledge about sex differences in health and illness exist and must be eliminated. Scientists conducting research on sex differences are confronted with an array of barriers to progress, including ethical, financial, sociological, and scientific factors.

After considering the data and examples presented throughout this report, the committee expects that the public, scientific, and policy communities alike will agree that the understanding of sex differences in health and illness merits serious scientific inquiry in all aspects of biomedical and health-related research. Some of the answers have been stumbled upon fortuitously. Until the question of sex, however, is routinely asked and the results-positive or negative-are routinely reported, many opportunities to obtain a better understanding of the pathogenesis of disease and to advance human health will surely be missed.

FINDINGS AND RECOMMENDATIONS

Every Cell Has a Sex

The biological differences between the sexes have long been recognized at the biochemical and cellular levels. Rapid advances in molecular biology have revealed the genetic and molecular bases of a number of sex-based differences in health and human disease, some of which are attributed to sexual genotype-XX in the female and XY in the male. Genes on the sex chromosomes can be expressed differently between males and females because of the presence of either single or double copies of the gene and because of several other phenomena: of different meiotic effects, X-chromosome inactivation, and genetic imprinting. The inheritance of either a male or a female genotype is further influenced by the source (maternal or paternal) of the X chromosome. The relative roles of the sex chromosome genes and their expressions explain X-chromosome-linked diseases and are likely to illuminate the reasons for the heterogeneous expression of some diseases within and between the sexes.

These findings argue that there are multiple, ubiquitous differences in the basic cellular biochemistries of males and females that can affect an individual's health. Many of these differences do not necessarily arise as a result of differences in the hormonal regime to which males and females are exposed but are a direct result of the genetic differences between the two sexes. Thus, the committee makes the following recommendation:

RECOMMENDATION 1: Promote research on sex at the cellular level.

The committee recommends that research be conducted to

determine the functions and effects of X-chromosome- and Y-chromosome-linked genes in somatic cells as well as germ-line cells;

develop systems that can identify and distinguish between the effects of genes and the effects of hormones.

Sex Begins in the Womb

Sex differences of importance to health and human disease occur throughout the life span, although their specific expression varies at different stages of life. Some differences originate in events occurring in the intrauterine environment, where developmental processes differentially organize tissues for later activation in the male or female. In the prenatal period, sex determination and differentiation occur in a series of sequential processes governed by genetic and environmental factors. During the prepubertal period, behavioral and hormonal changes manifest the secondary sexual characteristics that reinforce the sexual identity of the individual through adolescence and into adulthood. Hormonal events occurring in puberty lay a framework for biological differences that persist through life and contribute to the variable onset and progression of disease in males and females. It is important to study sex differences at all stages of the life cycle, relying on animal models of disease and including sex as a variable in basic and clinical research designs.

RECOMMENDATION 2: Study sex differences from womb to tomb.

The committee recommends that researchers and those who fund research focus on the following areas:

inclusion of sex as a variable in basic research designs,

expansion of studies to reveal the mechanisms of intrauterine effects, and

encouragement of studies at different stages of the life span to determine how sex differences influence health, illness, and longevity.

Sex is an important marker of individual variability. Some of this sex-related variability results from events that occur in the intrauterine environment but that do not materialize until later in life. Current research varies in its level of attention to these matters.

The committee acknowledges that inclusion of people, animals, or cells and tissues of both sexes in all studies is not always feasible or appropriate. Rather, the committee is urging researchers to regard sex, that is, being male or female, as an important basic human variable that should be considered when designing, analyzing, and reporting findings from studies in all areas and at all levels of biomedical research. Statistical methods can be used to evaluate the effect of sex without necessarily doubling the sample size of every study. In addition, it is particularly important that researchers revisit and revise approaches to studying whole-animal physiology in light of what has been learned in the past decade about major sex differences.

RECOMMENDATION 3: Mine cross-species information.

Researchers should choose models that mirror human sex differences and that are appropriate for the human conditions being addressed. Given the interspecies variation, the mechanisms of sex differences in nonhuman primates may be the best mimics for some mechanisms of sex differences in humans. Continued development of appropriate animal models, including those involving nonhuman primates, should be encouraged and supported under existing regulations and guidelines (see the Guide for the Care and Use of Laboratory Animals [National Research Council, 1996]).

Researchers should be alert to unexpected phenotypic sex differences resulting from the production of genetically modified animals.

Sex Affects Behavior and Perception

Basic genetic and physiological differences, in combination with environmental factors, result in behavioral and cognitive differences between males and females. Sex differences in the brain, sex-typed behavior and gender identity, and sex differences in cognitive ability should be studied at all points in the life span. Hormones play a role in behavioral and cognitive sexual dimorphism but are not solely responsible. In addition, sex differences in the perception of pain have important clinical implications. Research is needed on the natural variations between and within the sexes in behavior, cognition, and perception, with expanded investigation of sex differences in brain organization and function. To better understand the influences and roles of factors that may lead to sex differences, the committee makes the following recommendations:

Naturally occurring variations provide useful models that can be used to study the influences and origins of a range of factors that influence sex differences.

RECOMMENDATION 5: Expand research on sex differences in brain organization and function.

New technologies make it possible to study sex-differential environmental and behavioral influences on brain organization and function and to recognize modulators of brain organization and function. Innovative ways to expand the availability of and reduce the cost of new technologies need to be explored.

Sex Affects Health

Males and females have different patterns of illness and different life spans, raising questions about the relative roles of biology and environment in these disparities. Dissimilar exposures, susceptibilities, and responses to initiating agents and differences in energy storage and metabolism result in variable responses to pharmacological agents and the initiation and manifestation of diseases such as obesity, autoimmune disorders, and coronary heart disease, to name a few. Understanding the bases of these sex-based differences is important to developing new approaches to prevention, diagnosis, and treatment.

RECOMMENDATION 6: Monitor sex differences and similarities for all human diseases that affect both sexes.

Investigators should

consider sex as a biological variable in all biomedical and health-related research; and